1. Field of the Invention
This invention relates generally to an apparatus and method for delivery and vaporization of a liquid reagent for transport to a deposition zone, e.g., a chemical vapor deposition (CVD) reactor.
2. Description of the Related Art
In the formation of thin films, layers and coatings on substrates, a wide variety of source materials have been employed. These source materials include reagents and precursor materials of widely varying types, and in various physical states. To achieve highly uniform thickness layers of a conformal character on the substrate, vapor phase deposition has been used widely as a technique. In vapor phase deposition, the source material may be of initially solid form which is sublimed or melted and vaporized to provide a desirable vapor phase source reagent. Alternatively, the reagent may be of normally liquid state, or may be in a liquid solution or suspension, which is vaporized, or the reagent may be in the vapor phase in the first instance.
In the manufacture of advanced thin film materials, a variety of reagents may be used. These reagents may be used in mixture with one another in a multicomponent fluid which is utilized to deposit a corresponding multicomponent or heterogeneous film material. Such advanced thin film materials are increasingly important in the manufacture of microelectronic devices and in the emerging field of nanotechnology. For such applications and their implementation in high volume commercial manufacturing processes, it is essential that the film morphology, composition, and stoichiometry be closely controllable. This in turn requires highly reliable and efficient means and methods for delivery of source reagents to the locus of film formation.
Examples of advanced thin film materials include refractory materials such as high temperature superconducting (HTSC) materials including YBa.sub.2 Cu.sub.3 O.sub.x, wherein x is from about 6 to 7.3, BiSrCaCuO, and TlBaCaCuO. Barium titanate, BaTiO.sub.3, and barium strontium titanate ("BST"), Ba.sub.x Sr.sub.1-x TiO.sub.3, have been identified as ferroelectric and photonic materials with unique and potentially very useful properties in thin film applications of such materials. Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 is a photonic material whose index of refraction changes as a function of electric field and also as a function of the intensity of light upon it. Lead zirconate titanate, PbZr.sub.1-x Ti.sub.x O.sub.3, is a ferroelectric material whose properties are very interesting. The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are useful for scintillation detecting and coating of optical fibers. Refractory oxides such as Ta.sub.2 O.sub.5 are coming into expanded use in the microelectronics industry; Ta.sub.2 O.sub.5 is envisioned as a thin-film capacitor material whose use may enable higher density memory devices to be fabricated.
Thin films comprising the Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are potentially very useful as buffer layers for interfacing between silicon substrates and HTSC or GaAs overlayers or between GaAs substrates and HTSC or silicon overlayers, and combinations of two or all of such metal fluorides may be employed in forming graded compositions in interlayers providing close lattice matching at the interfaces with the substrate and overlayer constituents of the composite. For example, a silicon substrate could be coated with an epitaxial layer of BaF.sub.2 /CaF.sub.2, SrF.sub.2 /CaF.sub.2, or SrF.sub.2 /CaF.sub.2 /BaF.sub.2, whose composition is tailored for a close lattic to the silicon. If the ratio of the respective Group II metal species in the metal fluoride interlayers can be controlled precisely in the growth of the interlayer, the lattice constant could be graded to approach the lattice constant of GaAs. Thus, a gallium arsenide epitaxial layer could be grown over the metal fluoride interlayer, allowing the production of integrated GaAs devices on widely available, high quality silicon substrates. Another potential use of such type of metal fluoride interlayers would be as buffers between silicon substrates and polycrystalline HTSC films for applications such as non-equilibrium infrared detectors. Such an interlayer would permit the HTSC to be used in monolithic integrated circuits on silicon substrates.
BaTiO.sub.3 and Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 in film or epitaxial layer form are useful in photonic applications such as optical switching, holographic memory storage, and sensors. In these applications, the BaTiO.sub.3 or Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 film is the active element. The related ferroelectric material PbZr.sub.1-x Ti.sub.x O.sub.3 is potentially useful in infrared detectors and thin film capacitors well as filters and phase shifters.
Chemical vapor deposition (CVD) is a particularly attractive method for forming thin film materials of the aforementioned types, because it is readily scaled up to production runs and because the electronic industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming the thin films by CVD permits the integration of these materials into existing device production technologies. CVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having close crystal structures.
CVD requires that the element source reagents, i.e., the precursor compounds and complexes containing the elements or components of interest must be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The elemental component source reagent must decompose in the CVD reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved if the reagent can be delivered into the reactor in a controllable fashion. In this respect the reagents must not be so chemically stable that they are non-reactive in the deposition chamber.
Desirable CVD reagents therefore are fairly reactive and volatile. Unfortunately, for many of the refractive materials described above, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are elements, i.e., the Group II metals barium, calcium, or strontium, or the early transition metals zirconium or hafnium, for which no or few volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it therefore is very difficult to control the stoichiometry of the deposited films from such decomposition--susceptible reagents.
When the film being deposited by CVD is a multicomponent substance rather than a pure element, such as barium titanate or the oxide superconductors, controlling the stoichiometry of the film is critical to obtaining the desired film properties. In the deposition of such materials, which may form films with a wide range of stoichiometries, the controlled delivery of known proportions of the source reagents into the CVD reactor chamber is essential.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven difficult because of problems of premature decomposition or stoichiometry control. Examples include the deposition of tantalum oxide from the liquid source tantalum ethoxide and the deposition of titanium nitride from bis(dialkylamide)titanium reagents.
In recent years, the liquid delivery technique has come into increasingly widespread use, for chemical vapor deposition applications using "problematic" reagents such as those described above. In the liquid delivery approach, the reagent composition is dissolved or suspended in a suitable solvent medium, and the liquid is subjected to rapid vaporization to produce a precursor vapor which then is transported to the chemical vapor deposition chamber, optionally augmented by a carrier gas, e.g., an inert gas or an oxidant medium, to be contacted with the substrate heated to appropriate temperature. The liquid delivery technique has come into increasingly widespread use, and continues to evolve as a thin film materials fabrication methodology.
While source reagent liquid delivery systems present distinct advantages over conventional techniques, there is often some fraction of the precursor compound that decomposes into very low volatility compounds that remain at the vaporization zone. This deficiency is an important issue in the operation of CVD processes that use thermally unstable solid source precursors which undergo significant decomposition at conditions needed for sublimation. Such decomposition can occur in all reagent delivery systems that involve a vaporization step, including flash vaporizer liquid delivery systems as well as more conventional reagent delivery systems that include bubblers and heated vessels operated without carrier gas.
Although well-behaved CVD precursors vaporized under "ideal" conditions will form no deposits or residue at the vaporization zone, deviations from this situation are common and can be divided into several categories:
1) Reactive impurities in either the precursor or in the carrier gas decompose at the vaporizer temperatures.
2) Spatial and temporal temperature variations occur in the vaporization zone, with temperatures in some regions being sufficient to bring about decomposition.
3) CVD precursors are employed which are thermally unstable at the sublimation temperature.
Optimization of the conditions used in the vaporizer of reagent delivery systems can minimize the fraction of the delivered precursor that decomposes (and remains) at the vaporization zone, but virtually all solid and liquid precursors undergo some decomposition when they are heated for conversion to the gas phase, although this fraction is negligibly small in "well-behaved" compounds. Use of precursors that tend to decompose near their vaporization temperature may be mandated by availability (i.e., where the selected precursor possesses the best properties of available choices) or by economics, where precursor cost is strongly dependent on the complexity of its synthesis.
Additionally, CVD precursors often contain impurities, and the presence of those impurities can cause undesirable thermally activated chemical reactions at the vaporization zone, also resulting in formation of involatile solids and liquids at that location. For example, a variety of CVD precursors (such as tantalum pentaethoxide) are water-sensitive and hydrolysis can occur at the heated vaporizer zone forming tantalum oxide particulates that may be incorporated into the growing tantalum oxide film with deleterious effects.
Despite the advantages of the liquid delivery approach (which include improved precision and accuracy for most liquid and solid CVD precursors and higher delivery rates), the foregoing deficiencies pose a serious impediment to widespread use of the vaporization liquid delivery technique for providing volatilized reagent to the CVD reactor.
Improved liquid delivery systems are disclosed in U.S. Pat. No. 5,204,314 issued Apr. 20, 1993 to Peter S. Kirlin et al. and U.S. Pat. No. 5,536,323 issued Jul. 16, 1996 to Peter S. Kirlin et al., which describe heated foraminous vaporizer elements such as microporous disk elements, frits, screens, meshes, grids, porous sintered matrices, and other high surface area elements onto which the liquid solution or suspension is introduced for vaporization thereof. These heated foraminous vaporizer elements may be composed of various materials including ceramic, glass or metal and serve to carry out the rapid vaporization step. In use, liquid source reagent compositions are flowed onto the foraminous vaporizer element for flash vaporization thereof. Vapor thereby is produced for transport to the deposition zone, e.g., a CVD reactor. The liquid delivery systems of these patents provide high efficiency generation of vapor from which films may be grown on substrates. Such liquid delivery systems are also usefully employed for generation of multicomponent vapors from corresponding liquid reagent solutions containing one or more precursors as solutes, or alternatively from liquid reagent suspensions containing one or more precursors as suspended species.
The art continues to seek improvements in liquid delivery systems for vapor-phase formation of advanced materials, as well as improvements in ancillary equipment such as fluid transport, vaporizer, mixing, and control means associated with the liquid delivery system, and process conditions and techniques for operating the liquid delivery system and ancillary equipment in a maximally efficient manner.
One area in which improvement is sought relates to the time-varying thermal loading of the vaporizer in the liquid delivery system.
This variation is due to the fact that in a manufacturing environment the continuous flow of reagent(s) through the vaporizer, be it a liquid delivery system with a vaporizer element for volatilizing liquid source reagents contacted with the vaporizer element, or a conventional bubbler-based reagent vaporization system, in the absence of deposition downstream is not desirable. A reagent on/off sequence is advantageous both economically and environmentally in the liquid delivery system, with the reagent thereby being cyclically fed to the vaporizer element. Such reagent on/off sequence, however results in cooling and heating cycles occurring in the vaporizer, which upset the thermal gradients established in the vaporizer during its operation. This, in turn, adversely affects the reproducibility of the films deposited in the downstream chemical vapor deposition chamber.
It would therefore be a significant advance in the art of liquid delivery reagent vaporization systems, and is an object of the present invention, to provide an effective means of delivering a reagent liquid in vapor form to a chemical vapor deposition process with minimal formation of deposits and residue.
It is another objective of the present invention to provide a vaporizer arrangement which is not adversely affected by periodic feeding of source reagent liquid to the vaporizer element of the liquid delivery system.
It is a still further objective of the present invention to provide a liquid delivery reagent vaporization system having a periodic feeding of source liquid reagent to a vaporizer element without adversely affecting the thermal gradients established in the vaporizer during its operation.
Other objects and advantages of the invention will be more fully apparent from the ensuring disclosure and appended claims.